This invention relates to welding components of catheters and more particularly to welding balloons to catheters. Bonds provided by such welding must provide mechanical strength, lateral flexibility, and resistance to hydraulic leakage under high pressures.
Balloon catheters are used in the treatment of narrowings in tubular passageways of the human body, especially arterial obstructions, which are generally referred to as occlusions or stenoses. In particular, balloon catheters are used in procedures such as coronary angioplasty, peripheral angioplasty, drug delivery and stent delivery. During such procedures, a slender tubular catheter is threaded through the patient""s passageways until the remote, or distal end of the catheter reaches the treatment site. Fluid injected into the external, proximal end of the catheter flows through a hollow lumen to reach and expand a balloon mounted to the distal end of the catheter. The fluid pressure used is varied as required to open the narrowing with the balloon, deliver the stent or delivery a drug at the treatment site.
Construction of a balloon catheter typically requires mounting of a separately molded balloon to the distal end of the catheter. Such balloons have a generally cylindrical dilating portion with conical ends tapering to shorter, smaller diameter, cylindrical necks that fit closely around the distal portion of the catheter where they are attached. The dilating portion of the balloon is made as thin as possible to achieve the lowest possible profile when the deflated balloon is wrapped around the catheter, and excellent flexibility of the assembly for negotiating tortuous passageways, while maintaining a reliable burst pressure for the intended medical application. Historically, balloons have been formed of a thermoplastic polymeric material that is optically transparent to facilitate viewing air bubbles that are flushed out with inflation liquid as the catheter is prepared for use. These balloons are typically blow-molded radially outward from extruded tubing so that the cone portions, and especially the mounting necks, are thicker and less flexible than the larger diameter dilating portion. However, several balloon-making processes have been developed to provide cones and necks that are about as thin as the dilating portion of the balloon. Such ultra-thin necks are especially susceptible to thermal damage if heat welding is used to attach the balloon to the catheter.
When the balloons are bonded using adhesive between the necks and the catheter, the increased stiffness in this area can reduce the ability of the catheter to track through tight bends. Historically, the solution to this design problem has been to make the balloon necks and/or the bond lengths as short as possible because shorter stiff sections have a reduced effect on catheter trackability. Further improvements to the flexibility of balloon bonds included welding, or melt-bonding the balloon necks to the catheter. While welding improves the joint flexibility compared to the use of adhesives, it brings about new difficulties, including the sufficient control of heat to create a satisfactory bond without damaging the surrounding structure. The most significant damage caused by poorly controlled welding heat is stiffening of the balloon cones resulting from crystallization, which is a loss of desirable molecular orientation achieved during stretch blow-molding of the balloon. Thermal control is especially difficult in small balloon catheters such as those used in the treatment of coronary artery disease.
A known approach to heat bonding balloons is to place a section of heat-shrink tubing around the neck to be bonded, then to shrink the tubing by applying hot air. The heating not only shrinks the tubing to apply pressure to the assembly, but the shaft and balloon neck are also melted together. During this process, the cone portion and remainder of the balloon must be carefully insulated to avoid heat damage.
Another known method for welding balloons to catheters is to advance the assembled catheter shaft and the neck of the balloon into a heated mold having a tapered bore to compress the neck against the shaft during bonding. A low-mass mold may be quickly heated and cooled using radio frequency energy. A disadvantage of this process is that very thin balloon necks may peel back as the assembly moves into the mold.
Another known welding approach for dilatation balloon bonding uses laser energy focused in the area where the bond is desired, on the annular interface between the balloon neck and the catheter shaft. This narrowly focused energy solves the heat control problem, but the process requires the materials of both the catheter and balloon to have the same high absorptivity for the particular energy emitted by the laser source, which is in the far-infrared range. Thus, the designer""s choice of materials is limited. Alternatively, the known welding process may use a laser source having a wavelength in the red and near-infrared range, while still using balloon and catheter materials that were selected to strongly absorb far-infrared energy. This optional red and near-infrared energy is not well absorbed by the balloon and shaft materials in the bond area. To overcome this poor energy absorption, a component that is absorptive of red and near-infrared energy is placed in the bonding interface site between the balloon neck and catheter. The additional element absorbs energy sufficiently to melt the adjacent neck and shaft polymers, creating the weld. Thus, the known laser process solves some of the heat control problems in balloon welding, but requires a limited selection of particular pairs of polymers and, alternatively, the use of an extra weld element made of a material that is different from these particular polymer pairs.
It is an object of the present invention to provide a process for welding dilatation balloons to catheters with good control of the heat required.
Another object of the invention is to weld very thin balloon necks to an underlying catheter shaft with minimal thermal damage to portions of the balloon adjacent to the weld.
Another object of the invention is to provide a balloon angioplasty catheter wherein the balloon is welded to the catheter shaft, and the selection of polymer materials is broader than previously known for laser welded assemblies.
In accordance with the present invention, a balloon catheter and process for making said catheter are provided. The catheter comprises a slender, elongate, tubular, flexible shaft having proximal and distal ends and at least one lumen extending from the proximal end to the distal end. The balloon is specially designed to be mounted onto the distal end of the catheter shaft, and to be inflated in conformance with tubular passageways in the human body. The balloon has a generally cylindrical dilating portion with conical ends tapering to shorter, smaller diameter, cylindrical necks that fit closely around the distal portion of the catheter shaft where the necks are attached. Through-transmission welding is used to attach at least one balloon neck to the catheter shaft, thus providing a short, strong, leak-proof bond that adds minimal bending stiffness to the catheter assembly. The welding process includes the following steps:
a. mounting a balloon formed of a transparent or translucent thermoplastic polymer around the distal end of a catheter shaft formed of opaque thermoplastic polymer;
b. transmitting laser energy in the red and near-infrared wavelength range through a portion of the balloon neck to the underlying catheter shaft causing both polymers to melt in a cylindrical zone that includes the annular interface between the neck and shaft; and
c. permitting the molten polymers to cool, forming a solid weld joint.
The balloon may be formed from any thermoplastic polymer that is suitable for making catheter balloons and is transparent or translucent to energy in the red and near-infrared range. The catheter shaft may be formed from any material that is suitable for catheter construction and is opaque to the red and near-infrared energy range. Alternatively, the distal region of the catheter shaft may be made from a multi-layer coextrusion wherein only the outer layer needs to be opaque to the red and near-infrared energy range. Coextruded shaft construction offers further design flexibility, such as the use of a very low friction polymer for an inner layer, which may form a guidewire lumen. The energy directed toward the desired bond area is transmitted through the balloon neck without being absorbed significantly. It is the underlying catheter shaft, and particularly the outer shaft surface that absorbs the energy and rises in temperature. During the process, heat is conducted from the shaft such that both adjacent members melt in the weld area.
The preferred generator of red and near-infrared energy may be either a continuous ND:YAG laser, or a low power diode laser, either source having the following characteristics: a wavelength of 630-1580 nm; a spot size of approximately 580 micron (0.023 inches) diameter; and a power level of approximately 0.6-0.8 watts.
To obtain a short, annular weld between the balloon neck and the catheter shaft, it is preferred to have rotational relative motion between the assembled components and the energy source, most preferably by rotating the balloon and shaft combination about a central axis beneath a laser beam. Other relative motion processes are also possible, including processes which will form a short, helical, beam pattern within the desired annular bond area.
Thus, in accordance with the present invention, a balloon catheter is provided wherein the balloon is welded securely, yet flexibly to the catheter shaft, and the selection of polymer materials for both the balloon and the shaft is broader than previously known for laser welded assemblies. Any mutually heat bondable thermoplastic polymers may be selected, with the only limitations being that the balloon material is transparent or translucent and the shaft material is opaque, each with respect to red and near-infrared light. The balloon and shaft materials do not need to have matching, high absorptivity of far-infrared energy.
Another advantage of the invention is that very thin balloon necks can be welded to the catheter shaft with minimal damage to the necks or to the cone portions of the balloon because the laser beam can be transmitted through the transparent or translucent balloon material with little or no absorption of the welding energy.